In order to establish the composition of inorganic samples by means of combustion, in particular the composition of materials such as steel, iron, nonferrous metals, aluminum, titanium, zirconium, ores and alloys of the aforementioned substances, ceramic materials, cement, lime and the like, various methods and devices are known, with which a sample is in each case burned in an induction furnace and the combustion gases are analyzed automatically by means of corresponding detectors.
Besides the rapid (lasting less than approximately 1 to 2 minutes) combustion of samples by inductive heating of the sample, devices and methods are also known, for example from documents US 2008/0026471 A1 and U.S. Pat. No. 4,213,763, with which the samples to be analyzed are heated relatively slowly, specifically with heating rates of approximately 25-50° C. per minute, to temperatures in the range of approximately 600-850° C. in heated furnaces in which the furnace chamber itself is heated for example by resistance heating wires, wherein gases then have to be collected and analyzed over an accordingly long period of time. This slow heating process is indeed free from or only insignificantly tainted by specific problems, in particular such as the soiling of the furnace chamber described hereinafter by sample material and a conventional reaction accelerator to be added to the sample material, said accelerator boiling up upon sudden heating and spattering out from a sample crucible, however these devices and methods are not suitable for rapid sample analysis.
With devices and methods of the type concerned here, the heat required for combustion of the sample is generated by electromagnetic induction. The sample is arranged for this purpose in a sealable inner chamber of an induction furnace, said chamber being referred to hereinafter as a furnace chamber and being provided with gas inlets and gas outlets, wherein an electromagnetic high-frequency field for induction of eddy currents is then generated in the furnace chamber, generally by means provided outside the furnace chamber.
In order to obtain combustion gases that are concentrated to the highest possible extent over a short period of time and that can be utilized better by corresponding detectors compared to gases of lower concentration, and for economical reasons, the samples are to burn as quickly as possible. Hence, so called reaction accelerators are generally added to the samples, said accelerators having good coupling to the high-frequency field and therefore ensuring rapid heating of the sample.
In order to further promote the combustion process, the samples are blasted in the known devices with oxygen via a lance arranged above the sample to be burned. The oxygen blasted at a certain overpressure into the sealed furnace chamber is then advantageously used simultaneously as a carrier gas in order to transport the combustion gases from the furnace via a gas outlet and to the respective detectors. Here, the oxygen is normally to be fed not only via the lance, but also via further gas inlets provided in the upper and in the lower region of the furnace chamber, more specifically in such a way that a net gas flow is produced upwardly, where a gas outlet is arranged, which is connected to a sample line to transport the gases from the furnace chamber to corresponding detectors.
A big problem with the known devices is posed by the material spatters produced during the combustion process. The samples are generally placed into the furnace chamber in an open ceramic crucible and boil during the analysis process, wherein sample material and reaction accelerators spatter from the crucible and soil the furnace chamber, which is generally formed by a quartz glass pipe. The direct flow of oxygen onto the sample via a lance provided in the known devices increases the problem further. During subsequent combustion analyses, the spattered material is melted again, can become baked into the walls of the furnace chamber and in particular of a quartz glass pipe forming the furnace chamber and/or can falsify the measurement results. Baked spattered material results after a short time in destruction of the quartz glass pipe.
In order to prevent the spattered material from clogging the gas inlets and gas outlets provided in the respective furnace chamber, the gas inlets and gas outlets in the known combustion furnaces, with the exception of the lance arranged directly above the sample, are arranged as far as possible from the sample itself in the respective furnace chamber: This means however that the furnace chambers in the known combustion furnaces are relatively large, which entails a whole series of disadvantages. In the known devices, the combustion gases therefore diffuse initially in the entire furnace chamber and mix with the oxygen fed as carrier gas before they reach the gas outlet, and therefore the concentration of the combustion gases is low and only some of the available oxygen comes into contact with the sample.
The volume of the furnace chamber also influences the duration of a measurement cycle, since on the one hand it is to be ensured that the total amount of combustion gases is conveyed to the detectors where possible, and on the other hand the device has to be flushed with a gas, generally oxygen, between two successive measurements. The downtimes between two successive measurements in the known devices are typically between approximately 100 and 180 seconds, and the analysis times are typically between 120 and 140 seconds.
If the flow rate is increased, the analysis times are radically reduced (if the flow rate is doubled, the analysis time is approximately halved). However, an increase in the flow rate leads to a significant distortion of the measurement signal, whereby the measurement accuracy of the device is considerably lowered. For this reason, the flow rate is not increased in conventional devices.
In order to minimize the spatter formation, tungsten is generally used as a reaction accelerator. The tungsten couples well to the high-frequency field, melts quickly, and thus binds the sample material, which is generally in chip or powder form.
Approximately 1 g of tungsten is typically added to a sample quantity of approximately 0.5 to 1 g.
The use of tungsten entails a whole series of disadvantages however. During the combustion process, a fine tungsten oxide powder is produced, which soils the combustion furnace and the sample line. In the known devices, at least one filter for coarse particles and a filter for fine particles are therefore generally arranged in the sample line. Since tungsten oxide is harmful to health, specific precautionary measures must be taken when cleaning the combustion furnace and in particular the combustion chamber and the filter arrangement, and also when loading and unloading the furnace chamber. The tungsten oxide has to be disposed of separately. In addition, tungsten is expensive to purchase.
Theoretically, substances that can be handled more easily, such as pure iron in particular, could also be used as reaction accelerators. Pure iron couples very effectively to a high-frequency field in the furnace, is not harmful to health, and is additionally more cost effective compared to tungsten. However, substances such as pure iron spatter much more severely during heating compared to tungsten, and therefore considerably more and hotter particle spatters are produced, which bake more quickly into the furnace wall, clog gas inlets and gas outlets, and are difficult to remove. If the furnace chamber is formed by a quartz glass pipe, as is conventional, the service life of the quartz glass pipe is reduced so severely by the use of pure iron that the use of pure iron is not sensible from an economical point of view.